Fluid valves control the flow of fluid from one location to another. When the fluid valve is in a closed position, high pressure fluid on one side is prevented from flowing to a lower pressure location on the other side of the valve. Often fluid valves contain a movable fluid control member and a seat of some sort that cooperates with the fluid control member to control fluid flow through the valve. In some cases it may be desirable to characterize fluid as it flows through the valve, for example, to reduce noise. In these cases, a trim assembly may be used that includes a cage with a plurality of openings. The openings may be sized and shaped to characterize fluid flow through the trim assembly. However, the characterization of the fluid flow through the valve comes at the expense of a large pressure drop as fluid flows through the trim assembly.
In one example of a known fluid control valve, as illustrated in FIG. 1, a control valve 10 includes a valve body 12 having a fluid inlet 14 and a fluid outlet 16 connected by a fluid passageway 18. A trim assembly 20 is disposed within the valve body 12 between the fluid inlet 14 and the fluid outlet 16. The trim assembly 20 includes a cage 22 and a seat ring 24. A fluid control member, such as a plug 26 is disposed within the cage 22 and the plug 26 interacts with the seat ring 24 to control fluid flow through the valve body 12. A stem 28 is connected to the plug 26 at one end and an actuator 30 at another end. The actuator 30 controls movement of the plug 26 within the cage 22.
In some operations, a control valve is required to operate in a choked flow condition. Choked flow occurs when the velocity of fluid flowing through the control valve reaches supersonic speed (e.g., about 1070 feet per second for fuel flowing through the control valve for gas turbine electricity generation operations). In order to increase efficiency of choked flow control valves in gas turbine electricity generation, the choked flow condition should occur at a pressure drop of 10% or less of the inlet pressure. Flow obstructions, for example trim cages, cause the flow to choke more slowly and at higher pressure drops, which decreases the efficiency of the turbines. In order to solve this problem, current gas turbine engines include fuel control valves that have a threaded seat ring and a skirt guided plug to create a large unobstructed fuel flow region through the control valve.
In one example, as illustrated in FIG. 2, a known choked flow control valve 110 includes a valve body 112 having a fluid inlet 114 and a fluid outlet 116 connected by a fluid passageway 118. A trim assembly 120 is disposed within the valve body 112 between the fluid inlet 114 and the fluid outlet 116. The trim assembly 120 includes a skirt 122 and a seat ring 124. A fluid control member, such as a plug 126 is disposed at least partially within the skirt 122 and the plug 126 interacts with the seat ring 124 to control fluid flow through the valve body 112. The skirt 122 guides the plug 126 in reciprocating motion (e.g., up and down in FIG. 2) so that the plug 126 remains correctly aligned with the seat ring 124. A stem 128 is connected to the plug 126 at one end and an actuator 130 at another end. The actuator 130 controls movement of the plug 126 within the skirt 122. A large flow region 131 is created between the skirt 122 and the seat ring 124 so that flow obstructions through this region are minimized and the choked flow condition occurs at a relatively low pressure drop.
Known threaded seat ring choked flow control valve s suffer from many problems. For example, known threaded seat ring choked flow control valves suffer from inadvertent seat ring back out due to the shock waves generated by the supersonic flow. These shock waves cause intense vibrations, which can cause the seat ring to back out of its threaded and seated position, thereby causing flow disruptions and failure of the control valve.
Known threaded seat ring choked flow control valves also suffer from swirling flow through the gap between the skirt and the seat ring. This problem is caused by the change in flow corridor geometry from the inlet side of the skirt to the back side of the skirt. For example, as illustrated in FIG. 2, fluid flowing into the seat ring on the inlet side is relatively unobstructed as it enters the seat ring. However, flow entering the seat ring from the back side (opposite the inlet side) must change direction multiple times before entering the seat ring. This change in direction causes a change in flow velocity, which also changes the static pressure of the fluid relative to the static pressure of fluid flowing into the seat ring from the inlet side. This pressure differential causes the fluid to swirl as it flows through the seat ring, which reduces efficiency of the control valve and increases the pressure drop through the seat ring.
Furthermore, known threaded seat ring choked flow control valves are relatively expensive to manufacture, for example because the valve body threads must be machined, and are time consuming and difficult to manufacture.